It is known that at present, the average lifetime of a nuclear reactor is, of the order of some twenty years, as the continuous irradiation of the structures of such a facility and the fatigue of the materials make it impossible, in principle, to maintain the reactor in operation and, especially in production, beyond this period. Because of the very strong residual radioactivity which remains after the final shutdown of the reactor, it is still advisable to maintain certain parts of the latter, in particular the vessel which surrounds the core itself, after dismantling the latter by way of its elements, under suitable protection, in particular by leaving the vessel filled with water for a very long period.
However, it is understood that, under these conditions, it is necessary to monitor continuously certain characteristics of the metal material, generally special steel, which constitutes the vessel, in order to know how it behaves under prolonged irradiation under circumstances thus envisaged for the storage and prolonged preservation of the vessel. With this objective in mind, it is necessary to make use of specimens of the highly-irradiated metal constituting the wall of the vessel, enabling its transformation to be followed, in particular its rate of embrittlement which increases over time, in such a way as to inspect the behavior of the vessel with respect to pressure and temperature stresses which it continues to undergo because of the presence of the volume of water which it contains and of the vessel with respect to pressure and temperature stresses which it continues to undergo because of the presence of the volume of water which it contains and of the external climatic conditions. Moreover, the study of such specimens enables useful conclusions to be drawn as regards the improvements which can be made to the nature of the metal to be used for future reactors, by improving the safety conditions for the latter and above all the possible utilization times, in such a manner as to improve the return on the considerable financial investments which they represent.
Moreover, it is understood that the removal of significant specimens from within the very thickness of the vessel, via the inside of the latter, must be carried out with extreme precision, in such a way as not to undermine, beyond strict limits, the integrity of the vessel which remains filled with the protective water which has a certain activity and leakage of which towards the outside must be rigorously avoided.